Self-assembling reversible capsules

Figure 4-4 Schematic representations of various self-assembling reversible capsules derived from cavitand precursors.

Kang, J., Santamaria, J., Hilmersson, G. and Rebek, f. fr. (1998) Self-assembled molecular capsule catalyzes a Diels-Alder reaction. J. Am. Chem. Soc.. 120. 7389-7390. Kang, J., Hilmersson, G., Santamaria, J. and Rebek, f. fr. (1998) Diels—Alder reactions through reversible encapsulation. J. Am. Chem. Soc., 120, 3650—3656. 

The NMR spectroscopic investigations also showed that incarceration had a strong effect on the conformational preference of 89 (Fignre 9.29). Contrary to gas phase ab initio calculations, which predict trans-S9 as the lower energy conformation, the conformational equilibrium is reversed in the inner phase. Thns, confinement favors the coiled and more compact cis- over the extended trans conformation, which is consistent with the observations of coiled linear alkane conformations in self-assembled molecular capsules. ... 

The ultimate target of self-assembly of capsules and guest binding is the mimicry of what happens in nature. In particular, two phenomena are the source of inspiration the transport of molecules where the capsule acts as a reversible carrier that takes up the guest in a place and releases it in... 

Self-assembled molecular capsules are three-dimensional structures generated through reversible, noncovalent interactions such as hydrogen bonding between complementary subunits. They circumvent many problems of covalent capsules, such as multistep and complicated synthetic methods and lack of versatility. Therefore, recent researches are focused on self-assembled systans [2], Micelles, emulsions, vesicles, and noncovalent molecular capsules are examples of self-assembled NRs [2],... 

Self-assembled molecular capsules come in different shapes and cavity sizes and hold together through a variety of non-covalent forces they are capable of reversible encapsulation. Small molecule guests are entirely surrounded by larger molecular assemblies and the steric barriers imposed by a host retain the encapsulated molecules. 

In solution and in gas phase, the characterization of self-assembled cages/ capsules requires analytical methods operating in timescales that are in accordance with their hfetimes (milliseconds to hours). NMR spectroscopy and mass spectrometry using soft methods (ESI, MALDI) for vaporization and ionization are appropriate. In some cases, the containers structures and their complexes have also been characterized in the solid state by X-ray diffiraction. Generally, the purification of supramolecular containers and their encapsulation complexes using chromatography is not feasible owing to dynamic features of the reversible self-assembly process that led to their formation. 

Simultaneous, multiple ester formation was employed to assemble larger multitopic assemblies. For example, self-assembled molecular capsules via boronate esterification have been demonstrated. Two calix[3]arenes, one having the upper rim functionalized with diols, the other functionalized with boronic acids, were combined in the presence of a templating agent to form the heterodimeric capsule (Fig. 25). Extensive NMR analysis was performed to show that the two calixarenes in solution by themselves do not interact. It is only when tetramethyl ammonium acetate is added that the capsule forms. NMR diffusion studies have further demonstrated that the ammonium cation is included within the cavity. In addition, reversible decomposition-reconstruction of the capsule was observed in response to pH. 

Self-assembly has also been applied to numerous different classes of complexes in supramolecular chemistry [26]. Since we cannot discuss them all here, Fig. 1.3 shows only one example of a capsule reversibly formed from two identical self-complementary monomers which are bound to each other by hydrogen bonding... 

In fact, the large number of fascinating molecular architectures (Figure 5) formed from cucurbiturils lead to the concept of self-assembling capsules. These self-assembling capsules are receptors with enclosed cavities that are formed by the reversible noncovalent interaction of two or more, not necessarily identical, subunits. As would be in a subsequent section, the self-assembly of isolated calix[4]hydroquinone molecules leads to well-defined honeycomb architectures [64,65]. One of the advantages of these self-assembling capsules is that they can... 

Rebek has defined self-assembling capsules as receptors with enclosed cavities, formed by the reversible noncovalent interaction of two or more, not necessarily identical, sub-units [ll]. Consequently, the resulting capsule has a well-defined structure in solution, and shows binding capabilities that are absent for the individual components alone [11]. Examples provided in this section are restricted to capsules that form only in the presence of a template. As a result, unique sets of spectroscopic data are acquired for solutions of both the individual subunits in the presence and absence of the required templates. 

The reversible nature of self-assembling capsules has already been exploited as molecular switching devices and catalysts. Other applications may involve the development of artificial enzymes, and the design of specific drug delivery systems. 

Molecular capsules are structurally elaborated receptors that completely surround the hosted molecule(s). Encapsulation based on covalent bonds yields permanent arrangements of molecules-within-molecules. Reversible encapsulation, on the other hand, is based on self-assembling through formation of weak supramolecular bonds and offers possibilities for a dynamic in out exchange of encapsulated molecules. Most of the dimeric capsules developed by Rebek and his group are obtained through reversible self-assembly of resorcinarene subunits. When simultaneously encapsulated in the cylindri-cally shaped inner space of these capsules, two reactant molecules are temporarily isolated from others in solution and display reactivity features different from those in bulk solution. The matter has been extensively reviewed,and will not be discussed here. 

Metal-induced capsular self-assembly 6 (Fig. 3) of two deep, resorcinarene-based cavitands was described by Dal-canale and coworkers. Two tetracyanocavitands were connected through four square-planar Pd(II) or Pt(II) entities in CH2CI2, CHCI3, and aeetone. Evidence of encapsulation of one triflate anion upon dimerization was obtained ( F-NMR, x-ray analysis). For the Pt(ll) case, the assembly process was shown to be reversible Et N dissociated the capsule 6. while the addition of trifluoro-acetic acid restored it. 

Kobayashi investigated a self-assembled boronic ester cavitand capsule for photochemical reaction of 2,6-diacetoxyanthracene. Tsuda found a self-assembled helical anthracene nanofiber in a vortex. Chou examined rotational behaviors and fluorescence energy transfer of N-l- and N-2-anthryl succinimide derivatives. " Reversible photoinduced twisting of molecular crystal microribbons via [4 - - 4] photocycloaddition of 9-anthra-cenecarboxylic acid. Reversible single walled carbon nanotubes of 1,3-bis(9-anthracenylmethyl)imidazolium chloride was examined as a functionalized anthracene salts.Karatsu reported the intramolecular photodimerization of 9-substituted anthracene derivatives (253) tethered by oligosilanes giving [4 + 4] and [2 - - 4]cycloadducts (254), (255), and (256). ... 

The efficiency of these hemicarcerand and polycavitand nanocapsule syntheses approaches those of self-assembly processes using hydrogen bonding or metal-ligand coordination and results from selection of the least strained capsule through the thermodynamic control and reversibility of the system. 

As encapsulation within noncovalent self-assembled capsules is reversible. Depending on capsule entity, guest exchange inside and outside of it can occur via diffusion of the guest or (partial) dissociation of the capsule [5,7]. Encapsulation within molecular capsules is more than a physical entrapment. Interactions such as hydrophobic interactions, r- r interactions, or weak coordinative interactions play important roles in the selective encapsulation of particular guest molecules [7]. 

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